Flat bed laser readers, also known as horizontal slot scanners, have been used to electro-optically read one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, at a point-of-transaction workstation in supermarkets, warehouse clubs, department stores, and other kinds of retailers for many years. As exemplified by U.S. Pat. No. 5,059,779; U.S. Pat. No. 5,124,539 and U.S. Pat. No. 5,200,599, a single, horizontal window is set flush with, and built into, a horizontal countertop of the workstation. Products to be purchased bear an identifying symbol and are typically slid across the horizontal window through which a multitude of scan lines is projected in a generally upwards direction. When at least one of the scan lines sweeps over a symbol associated with a product, the symbol is processed and read.
The multitude of scan lines is generated by a scan pattern generator which includes a laser for emitting a laser beam at a mirrored component mounted on a shaft for rotation by a motor about an axis. A plurality of stationary mirrors is arranged about the axis. As the mirrored component turns, the laser beam is successively reflected onto the stationary mirrors for reflection therefrom through the horizontal window as a scan pattern of the scan lines.
It is also known to provide a point-of-transaction workstation not only with a generally horizontal window, but also with an upright or generally vertical window that faces an operator at the workstation. The upright window is oriented generally perpendicularly to the horizontal window, or is slightly rearwardly or forwardly inclined. The laser scan pattern generator within this dual window or bioptical workstation also projects the multitude of scan lines in a generally outward direction through the upright window toward the operator. The generator for the upright window can be the same as, or different from, the generator for the horizontal window. The operator slides the products past either window, e.g., from right to left, or from left to right, in a “swipe” mode. Alternatively, the operator merely presents the symbol on the product to an approximate central region of either window in a “presentation” mode. The choice depends on operator preference or on the layout of the workstation.
Each product must be oriented by the operator with the symbol facing away from the operator and generally towards either window of the bioptical workstation. Hence, the operator cannot see exactly where the symbol is during scanning. In typical “blind-aiming” usage, it is not uncommon for the operator to repeatedly swipe or present a single symbol several times before the symbol is successfully read, thereby slowing down transaction processing and reducing productivity.
The blind-aiming of the symbol is made more difficult because the position and orientation of the symbol are variable. The symbol may be located either low or high, or right or left, on the product, or anywhere in between, or on any of six sides of a box-shaped product. The symbol may be oriented in a “picket fence” orientation in which the elongated parallel bars of the one-dimensional UPC symbol are vertical, or in a “ladder” orientation in which the symbol bars are horizontal, or at any orientation angle in between.
In such an environment, it is important that the laser scan lines located at, and projected from, either window provide a full coverage scan zone which extends down as close as possible to the countertop, and as high as possible above the countertop, and as wide as possible across the width of the countertop. The scan patterns projected into space in front of the windows grow rapidly in order to cover areas on products that are positioned not on the windows, but several inches therefrom. The scan zone must include scan lines oriented to read symbols positioned in any possible way across the entire volume of the scan zone.
As advantageous as these laser-based, point-of-transaction workstations have been in processing transactions involving products associated with one-dimensional symbols each having a row of bars and spaces spaced apart along one direction, the workstations cannot process stacked symbols, such as Code 49 which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol, as described in U.S. Pat. No. 4,794,239, or PDF417 which increased the amount of data that could be represented or stored on a given amount of surface area, as described in U.S. Pat. No. 5,304,786, or two-dimensional symbols.
Both one- and two-dimensional symbols, as well as stacked symbols, can also be read by employing solid-state imagers which have a one-dimensional array or a single row, or a two-dimensional array or multiple rows, of cells or photosensors that correspond to image elements or pixels in a field of view of the imager. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, as well as associated circuits for producing electronic signals corresponding to the one- or two-dimensional array of pixel information over the field of view.
It is therefore known to use a solid-state imager for capturing a monochrome image of a symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use a solid-state imager with multiple buried channels for capturing a full color image of a target as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
It is also known to install the solid-state imager, analogous to that conventionally used in a consumer digital camera, in a bioptical, point-of-transaction workstation, as disclosed in U.S. Pat. No. 7,191,947, in which the dual use of both the solid-state imager and the laser scan pattern generator in the same workstation is disclosed. It is possible to replace all of the laser scan pattern generators with solid-state imagers in order to improve reliability and to enable the reading of two-dimensional and stacked symbols, as well as other targets.
However, it was thought that an overall imager-based reader would require about ten to twelve imagers in order to read a symbol that could be positioned anywhere on all six sides of a product. To be successful in the marketplace, as disclosed in commonly-assigned U.S. patent application Ser. No. 11/823,818, filed Jun. 28, 2007, the entire contents of which are incorporated herein by reference thereto, an all imager-based reader must eliminate the need for so many imagers to bring the cost of all the imagers, as well as the cost of each imager, down to an acceptable level, and it must also match, or at least be comparable to, the working range, processing speed, productivity and performance of a laser-based reader. In the case of a bioptical workstation having dual windows, the all imager-based reader must use similar window sizes and must also be able to scan anywhere across the windows and over a comparable working range as that of a laser-based reader, so that operators can achieve the high scanning productivity they have come to expect without any need to learn a new scanning technique.
As advantageous as the all imager-based bioptic reader can be in reading symbols, as disclosed in commonly-assigned U.S. patent application Ser. No. 12/220,333, filed Jul. 23, 2008, the entire contents of which are incorporated herein by reference thereto, interference or crosstalk among the imagers can occur if any two imagers are simultaneously operative. Each imager includes an illuminator for illuminating the symbol with illumination light from one or more illumination light sources, e.g., light emitting diodes (LEDs). A controller is operative for controlling each illuminator to illuminate the symbol, and for controlling each imager to capture the illumination light returning from the symbol over an exposure time period to produce electrical signals indicative of the symbol being read. Each illuminator is only operative during the exposure time period. The illumination light is typically folded by field mirrors to be reflected and captured through the windows.
If the exposure time periods from any two imagers are concurrent, then interference or crosstalk among the illuminators can be caused by multiple internal reflections from the field mirrors within the reader. The illuminated image being captured by any one imager may be corrupted by light associated with another imager. Also, the possibility of uneven illumination could occur if more than one set of illumination LEDs is energized at the same time. In addition, the peak current consumption of the entire reader may be too high if more than one set of illumination LEDs are energized at the same time.
To prevent such concurrent exposure time periods from any two imagers, preferably each having a wide VGA (WVGA) resolution, e.g., 800×480, 848×480, or 854×480, wider than VGA, it is known to configure each imager with a global shutter. With a global/synchronous shutter, all rows in the array are reset and then exposed simultaneously for a specified exposure time period. The global shutter is synchronized with the illuminator. The global shutter exposes the entire imager simultaneously. An entire frame is exposed and begins gathering light. When the specified exposure time period has elapsed, the imager stops gathering light and turns its current exposure into an electronic image. At the start of an exposure, the entire imager starts gathering light. At the end of the exposure, the light-gathering circuitry is electronically turned off, and the contents of the imager are then read out and processed to generate an image.
Although WVGA imagers with global shutters can work in the all imager-based bioptic reader, mega-pixel (MP) imagers are preferred, because they provide a much higher resolution, as well as an enhanced capability of reading symbols of high density. MP imagers with global shutters are available, but at a prohibitively high cost, especially when multiple imagers are required in the reader. MP imagers with rolling shutters are available at a much lower cost, but a rolling shutter exposes different rows or portions of the imager at different points in time, “rolling” through the frame. Different portions of the imager become light-sensitive at different moments in time, and this process sequentially proceeds down or across the course of the full frame, until the entire imager is exposed. This rolling action, however, can cause undesirable exposure at each imager to illumination from other illumination light sources from the other imagers, thereby degrading the captured image by such crosstalk. Accordingly, it would be desirable to use MP imagers with rolling shutters, but without the drawback of imager crosstalk.